---
id: javascript-object-properties
title: "JavaScript Object Properties"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["JS object properties", "dot notation", "bracket notation", "delete property", "in operator", "nested objects", "key value pairs"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.89
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "objects", "properties", "dot-notation"]
raw_sources: ["https://www.w3schools.com/js/js_object_properties.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Object Properties]]

## 🎯 한 줄 통찰 (One-line insight)
An object is a collection of `key:value` properties that can be changed, added, and deleted — accessed by dot notation, bracket notation, or an expression. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Properties are key:value pairs** — a JavaScript object is a collection of properties that can be changed, added, and deleted. [S1]
- **Three access ways** — dot notation, bracket notation, and expression (a name stored in a variable). [S1]
- **Dot notation preferred** — it is generally preferred for readability and simplicity. [S1]
- **Bracket notation when needed** — required when the property name is in a variable or is not a valid identifier (e.g. `"last-name"`). [S1]
- **`delete` removes value and property** — after deletion, accessing the property returns `undefined`. [S1]
- **`in` operator** — checks whether a property exists in an object. [S1]
- **Nested objects** — property values can be other objects, reachable by chaining dot/bracket notation. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Dynamic property access** — `person[myVariable]` resolves a property whose name is held in a variable. [S1]
- **Add-by-assignment** — assigning to a non-existent property (`person.nationality = "English"`) creates it. [S1]
- **Existence check before use** — `("firstName" in person)` guards access to optional properties. [S1]

## 📖 세부 내용 (Details)
**Properties are key:value Pairs**
A JavaScript object is a collection of properties. Properties can be changed, added, and deleted. [S1]

**Accessing JavaScript Properties**
You can access object properties by dot notation, bracket notation, or expression: [S1]
```javascript
// objectName.property
let age = person.age;

// objectName["property"]
let age = person["age"];

// objectName[expression]
let age = person[x];
```

**Dot Notation** — `objectName.propertyName`: [S1]
```javascript
person.firstname + " is " + person.age;
```

**Bracket Notation** — `objectName["propertyName"]`: [S1]
```javascript
person["firstname"] + " is " + person["age"];
```
In general, dot notation is preferred for readability and simplicity. Bracket notation is necessary in some cases: when the property name is stored in a variable (`person[myVariable]`), or when the property name is not a valid identifier (`person["last-name"]`). Bracket notation is useful when the property name is stored in a variable: [S1]
```javascript
let n1 = "firstName";
let n2 = "lastName";

let name = person[n2] + " " + person[n2];
```

**Changing Properties** — you can change the value of a property: [S1]
```javascript
person.age = 10;
```

**Adding New Properties** — add a new property by simply giving it a value: [S1]
```javascript
person.nationality = "English";
```

**Deleting Properties** — the `delete` keyword deletes a property from an object: [S1]
```javascript
const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
};

delete person.age;
```
```javascript
const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
};

delete person["age"];
```
The `delete` keyword deletes both the value and the property. After deleting, the property is removed; accessing it will return `undefined`. [S1]

**Check if a Property Exists** — use the `in` operator: [S1]
```javascript
const person = {
  firstName: "John",
  lastName: "Doe"
};

let result = ("firstName" in person);
```

**Nested Objects** — property values in an object can be other objects: [S1]
```javascript
myObj = {
  name:"John",
  age:30,
  myCars: {
    car1:"Ford",
    car2:"BMW",
    car3:"Fiat"
  }
}
```
You can access nested objects using dot notation or bracket notation: [S1]
```javascript
myObj.myCars.car2;
```
```javascript
myObj.myCars["car2"];
```
```javascript
myObj["myCars"]["car2"];
```
```javascript
let p1 = "myCars";
let p2 = "car2";
myObj[p1][p2];
```

**Summary**
Object properties are key:value pairs; access properties with dot notation or bracket notation; add, change, and delete properties using assignment and `delete`; use the `in` operator to check if a property exists. [S1]

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
| Access style | Use when | Note |
| --- | --- | --- |
| Dot notation `obj.prop` | Default | Preferred for readability and simplicity [S1] |
| Bracket notation `obj["prop"]` | Name in a variable, or not a valid identifier | e.g. `obj[myVar]`, `obj["last-name"]` [S1] |

## 🛠️ 적용 사례 (Applied in summary)
The page's own snippets are the canonical applied examples — accessing `person.firstname`/`person["firstname"]`, dynamic access via variables, `delete person.age`, the `in` check, and nested `myObj.myCars.car2` access. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Dynamic property access via a variable:
```javascript
let n2 = "lastName";
let name = person[n2];
```
Add then delete a property:
```javascript
person.nationality = "English";
delete person.age;
```
Existence check:
```javascript
let result = ("firstName" in person);
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.89
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Objects]], [[JavaScript Object Methods]], [[JavaScript Object Display]]
- **참조 맥락:** Referenced whenever reading, writing, adding, or removing data on an object.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Object Properties — https://www.w3schools.com/js/js_object_properties.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Object Properties" page (Astra wiki-curation, P-Reinforce v3.1 format).